1. Field of the Invention
The present invention generally relates to a method for fabricating a semiconductor device, and more particularly to a method for fabricating a thin film resistor.
2. Description of the Related Art
Resistors are devices used in mixed-mode integrated circuits. In terms of rectangular block resistors, resistance value (R) is in direct proportion to the length (L) of the rectangular block and is in inverse proportion to the cross-sectional area (A) of the rectangular block. Thus, resistance is calculated as R=ρ(L/A) where ρ is the resistivity of the material, L is the length of the resistor along the direction of the current and A is the cross sectional area of the resistor along the direction of the current.
In conventional practice, heavy dopants are applied to a portion designated as the bottom electrode of the transistor's capacitor and light dopants to a portion designated as the resistor on the same polysilicon layer. The top electrode of the capacitor and the gate electrode are formed on another polysilicon layers.
Lightly doped polysilicon thin film resistors are generally formed in the shape of a rectangle. Changing the doping concentration of the polysilicon layer allows the fabrication of resistors with different levels of resistivity. With the increased integration of semiconductor devices, requirements on the properties of materials used in semiconductor fabrication have also risen enabling devices to be formed in smaller dimensions with greater performance.
Polysilicon is the material used in the conventional method of fabricating thin film resistors. However, during the post-ion implantation annealing of high resistive polysilicon thin film resistors, doped material diffuses out increasing resistivity and lowering current flow, which makes it difficult to control the quality of the product. Additionally, the surface of the polysilicon layer may be oxidized during the subsequent thermal oxidation step reducing the effective dimension of the polysilicon layer. Moreover, the implanted dopants may also be consumed during the thermal oxidation causing imprecise resistivity of the thin film resistor.
FIGS. 1A-1D show the fabrication steps of a conventional high resistive, polysilicon thin film resistor and gate electrode. As shown in FIG. 1A, a shallow trench isolation region 102 is formed on a substrate 100 provided to isolate an active area 104. A polysilicon layer is deposited over the substrate using the chemical vapor deposition.
As shown in 1B, dopants 110, in the form of ions, are implanted into the polysilicon layer 106 to lower its resistivity.
As shown in 1C, after ion implantation 108 has been completed, the substrate is annealed in a chamber containing inert gas to restore the lattice structure and electrical type of the surface of the lightly doped polysilicon layer 106a causing the dopants implanted in the lightly doped polysilicon layer 106a to undergo thermal diffusion. Moreover, some of the dopants 110 will diffuse out increasing the resistivity of the lightly doped polysilicon layer.
As shown in FIG. 1D, the lightly doped polysilicon layer 106a is patterned to form a high resistive thin film resistor structure 106b above the shallow trench isolation region 102. This high resistive thin film resistor structure is an important part of the conventional, high resistive thin film resistor.
As shown in FIG. 1E, a thermal oxidation process is performed to form a gate oxide layer 112 on the substrate 100. The silicon oxide 116 is formed-on the surface layer of the high resistive thin film resistor structure during the thermal oxidation reducing the effective dimension of the high resistive thin film resistor 106b. Moreover, implanted dopants 110 are consumed. A patterned, doped polysilicon layer 114 is formed above the gate oxide layer 112. This patterned, doped polysilicon layer 114 serves as the gate layer for a metal oxide semiconductor transistor.
During the post-ion implantation annealing step described in the conventional practice above, dopants implanted in the polysilicon layer undergo a thermal diffusion. Consequently, some of the dopants diffuse out. Additionally, during the thermal oxidation step of the conventional practice described above, the surface of the lightly doped polysilicon layer oxidizes and becomes silicon oxide. As a result, the effective dimensions of the lightly doped polysilicon layer are changed. Moreover, during the process of thermal oxidation implanted dopants are consumed.
Thus, during the fabrication process of high resistive, polysilicon thin film resistors and gate electrodes, resistivity undergoes considerable change causing ineffective resistance.